p24 Antigen

This is a test used to check for infection with HIV after a recent exposure or to monitor your body's response to anti-HIV therapy. This test is done if you have been recently exposed to HIV. The p24 test identifies actual HIV viral particles in blood (p24 is a protein found in HIV). However, the p24 test is generally only positive from about one week to 3 - 4 weeks after infection with HIV. The p24 protein cannot be detected until about a week after infection with HIV because it generally takes that long for the virus to become established and multiply to sufficient numbers that they can be detected. The p24 proteins then become undetectable again after sufficient antibodies to HIV have been produced because they bind to the p24 protein and eliminate it from the blood. Once antibodies are produced, the p24 test will register negative even in people who are infected with HIV. At that point, the regular HIV antibody test will then be positive. Later in the course of HIV, p24 protein levels again become detectable.

A p24 test may be ordered if you have been recently exposed to HIV and wish to know your HIV status. It may also be ordered if you are donating blood or if you already have HIV and your doctor wants to monitor your response to therapy or to determine if HIV is progressing into AIDS.


A blood sample is obtained by inserting a needle into a vein in the arm or blood may be taken from a finger stick.


No evidence of HIV antigen or antibodies.

Ranges for normal findings may vary among different laboratories and hospitals. You should always check with your doctor after having lab work or other tests done to discuss the meaning of your test results and whether your values are considered within normal limits.


Your caregiver will go over the test results with you and discuss the importance and meaning of your results, as well as treatment options and the need for additional tests if necessary.


It is your responsibility to obtain your test results. Ask the lab or department performing the test when and how you will get your results.

The only reliable way to tell if you are infected with HIV is to get tested. This is because many people who have HIV do not experience symptoms for many years, and the symptoms they experience can be very similar to symptoms of other illnesses. For more information on symptoms of HIV infection, visit the CDC's web site.